Radio resource allocation for cellular wireless networks

ABSTRACT

Embodiments of the invention relate to cellular wireless networks and are particularly suited to networks including different types of base stations. So-called femtocell types of base stations are typically deployed within a subscriber&#39;s premises and operate at low transmit power, providing a very limited area of wireless coverage. A femtocell is typically deployed within the area of wireless coverage of a conventional macrocell, occupying the same frequency spectrum and timeslots as the macrocell. A problem can be presented to a user equipment terminal that is close to the femtocell but unable to gain access to it, because the transmissions from the femtocell may appear as interference to the user equipment terminal, preventing it from accessing the macrocell which it could otherwise access. A cellular wireless network according to an embodiment of the invention employs a method of allocating radio resource to femtocells so that the transmissions from femtocells do not occupy the same radio resource blocks as those used by the macrocell for signalling; embodiments of the invention thereby prevent interference associated with signalling to cause a connection to be lost, or prevent a connection being set up.

FIELD OF THE INVENTION

The present invention relates generally to cellular wireless datacommunications networks, and more specifically to method and apparatusrelating to a frequency arrangement for base stations such asfemtocells.

BACKGROUND OF THE INVENTION

The concept of the home deployed base station, or femtocell, isconsidered of increasing importance for cellular network operators.Femtocells operate at low downlink transmit power, and are designed toimprove the cellular coverage within a home or enterprise environmentand their immediate surroundings. Typically a femtocell would be linkedinto the wider cellular Radio Access Network through a customer'sbroadband link (e.g. digital subscriber line, cable, passive opticalnetwork or other wireline access technology), and provide user equipmentterminals with access to data.

The term “base station” is used here to refer to a radio transceiverconnected to a telecommunications network; a cell site may have severalbase stations, each serving a different area of wireless coverage. Thisdeployment of multiple base stations at a cell site is particularlycommon for macrocellular networks, whereas typically femtocell basestations are intended to be deployed individually, and accordingly areequipped with an omni-directional antenna. The user equipment terminal,often a mobile device such as a smart phone, Personal Digital Assistant(PDA) or laptop and the like, is alternatively referred to as a “userequipment”.

The use of femtocells is particularly applicable in high capacity packetdata cellular wireless communication systems such as HSPA (‘High SpeedPacket Access’), a so-called third generation evolutionary system, andLTE (Long Term Evolution), often referred to as a fourth generation (4G)system. Services using such systems can typically accommodate a variabledata rate to and from the user equipment, and can exploit a greater datarate should it be available, for example for the faster transfer of datafiles. It is accordingly advantageous to maximise the data capacityavailable to a user, and to this end adaptive modulation and coding istypically employed. The provision of a femtocell within a subscriber'spremises can provide a high capacity link within a small local area thatwill typically also be within the coverage area of a macrocell.

Although generally placed indoors, femtocells operate within an existingconventional cellular wireless network, which is termed a macrocellularnetwork. There may typically be hundreds of femtocells for everymacrocell. The large number of femtocells may interfere with the signalfrom the macrocells, particularly in the downlink direction from themacrocell base station to the user equipment, in some cases preventingaccess altogether. This problem is accentuated in the case of “closedaccess” femtocells which can only be used by a limited group of userequipments. User equipments outside the closed access group may receivea strong signal from the femtocell, however as they cannot use it, itacts as interference to signals received from macrocells.

FIG. 1 illustrates the problem of interference in the closed accesscase. A femtocell base station 12 is in communication with a userequipment 16. Nearby, a second user equipment 14 receives a strongsignal from the femtocell 12 but cannot establish a connection with thefemtocell, as the femtocell is closed to access by the second userequipment 14. As mentioned above, the signal from the femtocell basestation 12 conventionally occupies the same frequency band as isoccupied by the macrocell base station 10, so that the signal from thefemtocell base station 12 potentially acts as interference with signalsreceived from macrocell base station 10 at a second user equipment 14.

A second problem relating to the provision of a large number offemtocells within the coverage area of a macrocell base station is theexpenditure of power by a user equipment when performing measurementsfor handover decisions: it can be expected that there is a larger numberof near neighbours than are present in a conventional macrocell system,and this will trigger a commensurately larger number of handover-relatedactions on the part of the user equipment than is experienced inmacrocell systems. These actions involve processing on the part of theuser equipment, which is particularly undesirable given that the batterylife of a user equipment, typically a handset, should be maximised.

One known solution to these problems is for femtocells to use adifferent frequency channel to that used by a macrocell deployed in thesame area. Whilst this avoids interference problems, it is undesirableto operators, since spectrum is expensive to acquire.

Another known alternative is to use an interference mitigationtechnique: femtocells base stations detect the level of interferencethey are causing to the user equipments served by a macrocell basestation operating in the same area, and reduce their power accordingly.However, this will potentially limit the coverage area and data rateavailable to users of the femtocell.

It is an object of the present invention to provide a method andapparatus which addresses these disadvantages.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention, there isprovided a method of allocating radio resources in a radiocommunications network, the radio communications network comprising afirst base station open for access to substantially any user equipmentterminal of the radio communications network and a second base stationopen for access to only a predetermined one or more user equipmentterminals of the radio communications network, wherein the first basestation is arranged to use a first plurality of radio resource blocksfor radio communications with user equipment terminals, and the firstplurality includes one or more radio resource blocks useable by thefirst base station for signalling, the method comprising:

allocating a second plurality of radio resource blocks for use by thesecond base station in radio communications with user equipmentterminals, wherein the first plurality and the second plurality of radioresource blocks have at least one radio resource block in common, andwherein the second plurality of radio resource blocks does not includesaid one or more radio resource blocks useable by the first base stationfor signalling.

In embodiments of the invention, a first base station, such as amacrocell base station, is open to access to substantially any user andmay operate in all or part of the frequency band used by a second basestation, such as a femtocell, this being open for access to only apredetermined one or more user equipment terminals. A user equipmentterminal that is denied access to the femtocell base station may belocated in close proximity thereto, so that it may receive a strongersignal from the femtocell base station than from the macrocell basestation. The signal from the femtocell base station therefore haspotential to cause interference at the user equipment terminal,potentially preventing it from communicating with the macrocell basestation if interference is experienced with radio resource blocks thatare used by the macrocell base station for signalling. Since thefemtocell base station is allocated radio resource blocks other thanthose used for signalling by the macrocell base station, the userequipment terminal can advantageously maintain communication with themacrocell base station.

Radio resource blocks represent allocations of parts of the frequencyspectrum within specified timeslots. Preferably the radio resourceblocks allocated to the femtocell and macrocell base stations areseparated in frequency, with the benefit that transmission at thefemtocell and macrocell base stations can be simultaneous, thussimplifying the design as some timing constraints are removed.

Conveniently, the radio resource blocks allocated to the macrocell basestation occupy a contiguous frequency range, with the benefit that theavailable data capacity within a receiver bandwidth is maximised.Similarly, it is beneficial for the radio resource blocks allocated tothe macrocell base station to occupy a contiguous frequency range.

Advantageously, the radio resource blocks allocated to the femtocellbase station are a subset of the radio resource blocks allocated to themacrocell base station, with the benefit that no additional frequencyspectrum is required for the operation of the femtocell base stationbeyond that allocated for the operation of the macrocell base station.

In a further arrangement the radio communications network includes afurther base station, for example of the femtocell type, and thus onewhich is accessible by only a predetermined one or more user equipmentterminals. Preferably radio resource blocks allocated to this furtherfemtocell are not used by either the first femtocell or macrocell forsignalling so that interference with signalling by each of the basestations is prevented.

Further features and advantages of the invention will become apparentfrom the following description of preferred embodiments of theinvention, given by way of example only, which is made with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating potential interference from afemtocell base station received at a user equipment communicating with amacrocell base station;

FIG. 2 is a schematic diagram showing a conventional frequency divisionduplex frequency plan;

FIG. 3 a is a schematic diagram showing a conventional frequencyallocation for an uplink showing critical portions;

FIG. 3 b is a schematic diagram showing a conventional frequencyallocation for a downlink showing critical portions;

FIG. 3 c is a schematic diagram showing a conventional frequencyallocation for an uplink and downlink at baseband showing criticalportions;

FIG. 4 is a schematic diagram showing a frequency allocation for afemtocell relative to the frequency allocation to a macrocell accordingto an embodiment of the invention;

FIG. 5 is a schematic diagram showing a first example of a radioresource block allocation according to an embodiment of the invention;

FIG. 6 is a schematic diagram showing a second example of a radioresource block allocation according to an embodiment of the invention;

FIG. 7 is a schematic diagram showing radio resource block allocation toa macrocell and a femtocell according to an embodiment of the invention;

FIG. 8 is a schematic diagram showing the coverage area of a macrocelland five femtocells configured according to an embodiment of theinvention; and

FIG. 9 is a schematic diagram showing a frequency allocation to amacrocell and five femtocells configured according to an embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the invention will be described in the context ofa cellular wireless communication network comprising macrocell andfemtocell base stations, with particular reference to the frequencydivision duplexed systems. However, it will be understood that thisexample is for illustration purposes and that the invention can beapplied to radio communications generally and to systems complying withother wireless standards. For example, the invention is applicable toradio access systems generally and is applicable to time division duplexsystems in addition to frequency division duplexed systems.

FIG. 2 illustrates the frequency plan for a conventional frequencydivision duplex radio communication system, such as the Third GenerationPartnership Project Long Term Evolution System, know as LTE. It can beseen that a block of frequencies 18 with a centre frequency f_(cu) 22 isallocated for use in the uplink, that is the path from a user equipmentto a base station, and that a further block of frequencies 20 with acentre frequency f_(cu) 24 is allocated for use in the downlink, that isthe path from a base station to a user equipment. The centre frequenciesof the uplink and downlink blocks are separated by a frequencydifference f_(d).

FIG. 3 a illustrates the conventional resource allocation within theuplink frequency band. Certain frequency blocks are allocated for thecommunication of signalling information; these blocks 26 a and 26 b areshown as shaded portions. The remainder of the band 28 is allocated forthe communication of payload data. The position of the frequency blocksallocated to signalling in the uplink and the downlink are shown is anexample only; the position within the band may differ. Also, it shouldbe noted that data may be carried in addition in the frequency blocksallocated for signalling in both the uplink and the downlink.

FIG. 3 b illustrates the conventional resource allocation within thedownlink frequency band. Frequency block 26 is allocated for thecommunication of signalling information and the remainder of the band 28a, 28 b is allocated for the communication of payload data. Note that inthis example, the frequency blocks allocated in the uplink and downlinkfor signalling occupy different parts of the spectrum, relative to thecentre frequency.

FIG. 3 c shows the uplink and downlink frequency allocations overlaidfor comparison; in this case the frequency allocations are referred tobaseband, that is to say the centre frequency is translated to zero. Itcan be seen that the parts 26 a, 26 b and 26 c of the spectrum allocatedto signalling on either the uplink or the downlink occupy regions ateither end and the centre of the spectrum.

FIG. 4 illustrates a frequency arrangement according to an embodiment ofthe invention. It can be seen that a frequency band 30 is allocated foruse by a femtocell and that this band 30 does not overlap the regions 26d, 26 e and 26 f used for signalling in the macrocell spectrum 32. Thefrequency allocation to the femtocell is preferably a contiguous region,as shown in FIG. 4; whilst this is beneficial in terms of reducing therequirements on receiver bandwidth at the femtocell, the allocationcould be distributed in any manner across the spectrum 32, provide thatthere is no overlap between the frequency band 30 utilised by thefemtocell and signalling frequencies 26 d, 26 e, 26 f. The frequencyallocation illustrated in FIG. 4 shows the uplink and downlink bandsoverlaid at baseband; the translation to radio frequency will beperformed in such a way that the relationship between femtocell andmacrocell bands is maintained as illustrated.

The factors determining frequency allocation according to embodiments ofthe invention will now be described. In general, interference with datamessages can generally be tolerated because transmissions can occurdespite the interference, whereas interference with signalling messagesmay result in a dropped connection or the inability to establish aconnection to a base station at all. It is therefore preferable to avoidinterference with signalling messages when designing frequencyallocation schemes.

As stated above, interference with data messages can be toleratedbecause mitigation techniques such as error correction coding, orresending of corrupted data, can be employed to ensure that the messageis successfully received. In addition, provided parts of the band do notcontain interference, these can be used to transport data. There is alsoan efficient technique available known as hybrid ARQ (automatic resendrequest) that can mitigate the effects of corruption of data. In manycases, a reduced data rate may be tolerated by a user, or additionaltimeslots may be allocated to the user to compensate for the poorerreceived signal quality. Furthermore, retransmissions of the data to theuser equipment may be scheduled so as to select a portion of thespectrum unaffected by the interference due to the femtocell.

Turning now to signalling messages, such messages may comprise broadcastmessages allocating radio resource and enabling synchronisation, as isknown in the art. These messages are typically transmitted on thedownlink and affect operation of the links in both directions.Furthermore the messages cannot generally be reallocated to other partsof the band in the event that received signals experience interference.It is thus beneficial to position the femtocell frequency allocation inparts of the band that do not correspond with the signalling frequenciesof the macrocell downlink, to avoid interference from nearby femtocells.The frequency allocation to the femtocell also optionally avoids theportions of the uplink frequency allocation that contain signallinginformation.

FIG. 5 illustrates the radio resource allocation in the downlink interms of both frequency and time. It can be seen that in the frequencyband allocated for signalling, only the radio resource blocks indicatedby the reference numerals 26 a, 26 b are actually allocated tosignalling, and that this signalling region is shared by at least oneblock 28 c for the carriage of data. As a result it can be seen thatonly certain timeslots of the frequency spectrum nominally allocated tosignalling are used for signalling. This arrangement represents a morecomplex allocation of radio resources than is associated withconventional arrangements, since resources are allocated in time inaddition to being allocated in frequency. As a result femtocells canoccupy radio resource blocks that are not used by the macrocell forfrequency. Analogous regions in the radio resource blocks that areallocated to the uplink can also be allocated to a femtocell whileavoiding the parts of the radio resource used by the macrocell forsignalling.

FIG. 6 shows an alternative allocation of macrocell radio resourceblocks to that shown in FIG. 5; the regions shown are illustrative onlyand the position may vary between implementations. It can be seen thatregions 26 a, 26 b, 26 c, 26 d carrying signalling occupy potentiallyall of the frequency allocation at some point in time, meaning that animplementation in which interference with signalling is avoided byfrequency allocation alone is thus not feasible, and as a result anallocation of radio resource blocks to femtocells in both frequency andtime is required. Turning to FIG. 7, such an allocated region isindicated by part 42. An allocation such as this may be particularlyapplicable to some implementations of the IEE802.16 WiMax systems.

FIG. 8 shows a situation in which many femtocells 12 a . . . 12 e aredeployed in the area of wireless cellular coverage 34 of a macrocellbase station 10. The areas of wireless cellular coverage 36 of one ofthe femtocells 12 a is shown, and equivalent areas of wireless cellularcoverage are shown for the other femtocells in the illustration. It canbe seen that there is potential for interference between femtocells,especially in the case of femtocells indicated by reference numerals 12c, 12 d and 12 e.

The femtocells 12 a . . . 12 e and macrocell 10 are in potentialcommunication through the backhaul links 33 a . . . 33 e to atelecommunications network 31 and to a mobility management entity 38.The mobility management entity 38 may in an example of an implementationmanage the radio resource allocation to the femtocells as described inthis embodiment. This may involve allocating femtocells different radioresource blocks from those allocated to their neighbours to reduce theprobability of interference between femtocells.

FIG. 9 illustrates an example of an allocation of frequency bands 30 a .. . 30 f to multiple femtocells 12 a . . . 12 f operating in the area ofwireless coverage of a macrocell 10 occupying a frequency band indicatedby the reference numeral 32. Preferably a frequency allocation is madeto femtocells that does not overlap between femtocells. If this is notpossible, then at least the parts of the femtocell allocations used forsignalling should not overlap with the equivalent parts allocated toother adjacent femtocells.

Preferably at least part 40 of the portion of the macrocell frequencyspectrum that is allocated to payload data is protected from also beingallocated to femtocells, so that the macrocell retains a reasonable datacapacity when experiencing interference from femtocells.

It can be seen that the centre frequencies fc_f1, fc_f2, fc_f3, fc_f4,fc_f5 allocated to femtocells differ between respective femtocells.Since the algorithms controlling handover operate in such a way thathandover of a user equipment between base stations with different centrefrequencies are controlled by default by the network controller ratherthan locally at the user equipment, this has the advantage of enablingsuch handover operations to be inhibited if necessary by a networkcontroller. As a result the network controller can prevent a userequipment terminal making unnecessarily frequent handover measurementsthat would otherwise consume power and reduce battery life.

The above embodiments are to be understood as illustrative examples ofthe invention. It is to be understood that any feature described inrelation to any one embodiment may be used alone, or in combination withother features described, and may also be used in combination with oneor more features of any other of the embodiments, or any combination ofany other of the embodiments. Furthermore, equivalents and modificationsnot described above may also be employed without departing from thescope of the invention, which is defined in the accompanying claims.

The invention claimed is:
 1. A method of allocating radio resources in aradio communications network, the radio communications networkcomprising a first base station open for access to substantially anyuser equipment terminal of the radio communications network and a secondbase station open for access to only a predetermined one or more userequipment terminals of the radio communications network, wherein thefirst base station is arranged to use a first plurality of radioresource blocks for radio communications with user equipment terminals,and the first plurality includes one or more radio resource blocksuseable by the first base station for signalling, the method comprising:allocating a second plurality of radio resource blocks for use by thesecond base station in radio communications with user equipmentterminals, wherein the first plurality and the second plurality of radioresource blocks have at least one radio resource block in common, andwherein the second plurality of radio resource blocks does not includesaid one or more radio resource blocks useable by the first base stationfor signalling.
 2. A method according to claim 1, wherein eachrespective radio resource block is associated with a differentfrequency.
 3. A method according to claim 2, wherein the first pluralityof radio resource blocks occupies a contiguous frequency range.
 4. Amethod according to claim 2, wherein the second plurality of radioresource blocks occupies a contiguous frequency range.
 5. A methodaccording to claim 1, wherein the second plurality of radio resourceblocks is a subset of the first plurality of radio resource blocks.
 6. Amethod according to claim 1, wherein the one or more of said radioresource blocks useable by the first base station for signalling includeradio channels for synchronization, broadcast and for transmittingcontrol signalling.
 7. A method according to claim 1, wherein the radiocommunications network comprises a further base station open for accessto only a predetermined one or more user equipment terminals, the methodfurther comprising allocating a further plurality of radio resourceblocks for use by the further base station for radio communications withuser equipment terminals, wherein the first and further pluralities ofradio resources share at least one radio resource block, wherein thefurther plurality of radio resources excludes said one or more of saidradio resource blocks useable by the first base station for signalling,and wherein the second plurality of radio resource blocks and thefurther plurality of radio resource blocks includes one or more radioresource blocks different than those useable by the second and furtherbase stations respectively for signalling.
 8. A node of a radiocommunication network arranged to allocate radio resources in a radiocommunication network, the radio communications network comprising afirst base station open for access to substantially any user equipmentterminal supported by the radio communications network and a second basestation open for access to only a predetermined one or more userequipment terminals of the radio communications network, wherein thefirst base station is arranged to use a first plurality of radioresource blocks for radio communications with user equipment terminals,and the first plurality includes one or more radio resource blocksuseable by the first base station for signalling, the node beingarranged to: allocate a second plurality of radio resource blocks foruse by the second base station in radio communications with saidpredetermined one or more user equipment terminals, wherein the firstplurality and second plurality of radio resources blocks have at leastone radio resource block in common, and wherein the second plurality ofradio resources blocks does not contain said one or more of said radioresource blocks useable by the further base station for signalling.
 9. Abase station of a radio communications network, the base station beingopen for access to substantially any user equipment terminal of theradio communications network, the radio communications network furthercomprising a further base station, arranged to be open for access toonly a predetermined one or more user equipment terminals, said basestation being arranged to use a first plurality of radio resource blocksfor radio communications with user equipment terminals, the firstplurality of radio resource blocks including one or more radio resourceblocks useable by said base station for signalling, said base stationbeing arranged to: allocate a second plurality of radio resource blocksfor use by the further base station for radio communications with saidpredetermined one or more user equipment terminals, wherein the firstplurality and second plurality of radio resource blocks share at leastone radio resource block, and wherein the second plurality of radioresource blocks does not contain said one or more of said radio resourceblocks useable by said base station for signalling.
 10. A base stationof a radio communications network, the base station being arranged to beopen for access to only a predetermined one or more user equipmentterminals, the radio communications network further comprising a furtherbase station arranged to be open for access to substantially any userequipment terminal of the radio communications network, and beingarranged to use a first plurality of radio resource blocks for radiocommunications with user equipment terminals, the first plurality ofradio resource blocks including one or more radio resource blocksuseable by the further base station for signalling, wherein said basestation is arranged to: allocate a second plurality of radio resourceblocks for use by said base station for radio communications with saidpredetermined one or more user equipment terminals, wherein the firstplurality and second plurality of radio resource blocks share at leastone radio resource block, and wherein the second plurality of radioresource blocks does not contain said one or more of said radio resourceblocks useable by the further base station for signalling.